Microstructural Alteration of Structural Alloys by Low Temperature Irradiation with High Energy Protons and Spallation Neutrons

摘要

Several candidate alloys (nickel-based Alloy 718, iron-based 316 L and 304 L stainless steels) have been exposed to mixed proton-neutron particle fluxes and spectra at the Los Alamos Neutron Science Center (LANSCE), with the radiation conditions similar to those expected in the proposed Accelerator Production of Tritium (APT) facility. All alloys developed a high density of small black-spot damage and larger Frank loops, even at the lowest dose examined, producing rapid hardening and concurrent loss of ductility. The radiation-induced evolution of both microstructure and mechanical properties in the 300-series stainless steels is relatively simple, whereas that in Alloy 718 is rather more complex,. Superlattice spots corresponding to the age-hardening precipitate phases γ' and γ'' are lost from the diffraction patterns for Alloy 718 by only 0.6 dpa, the lowest protoneutron-induced dose level achieved in this experiment. Examination of neutron-irradiated samples to doses of only ～0.1 dpa showed that precipitates are faintly visible in diffraction patterns but are rapidly becoming invisible. It is proposed that the γ' and γ'' first become disordered (by <0.6 dpa), but remain as solute-rich aggregates that still contribute to the hardness at relatively low dpa levels, and then are gradually dispersed at higher doses. The two 300-series stainless steels (SS) appear to exhibit essentially identical evolution both in their microstructure and mechanical properties. In the 300-series stainless steels, there is an initial strong decrease in uniform elongation at very low dose levels, followed by a second, rather abrupt contribution to ductility loss at higher doses (>3dpa) that is not accompanied by any observed new or enhanced microstructural development. This may be a manifestation of gas-induced loss of ductility resulting from large levels of hydrogen and helium generated by irradiation in these alloys. Although the retained gas levels approached ～1 at% at the highest exposure levels, no discernible cavities were observed in any of the alloys. With one significant exception, all observed features of radiation-affected properties can be explained in terms of the observed developments in irradiated but undeformed microstructure. Explanation of the remaining feature will require observation of post-deformation microstructure.